Silverlight 4 WCF Polling Duplex doesn't work in IIS Express

Question

I have a silverlight 4 chat program that uses a WCF service with pollingDuplexHttpBinding.

The program works OK In: * Cassini * IIS Local Web Server * All non-Duplex WCF Services work fine in any setting

But it does NOT work in IIS 7.5 Express. A call is made but a 500 HTTP error is given, and nothing happens (no exceptions are thrown, program just ignores it)

Here's my Web.Config

<configuration>
<system.web>

    <compilation debug="true" strict="false" explicit="true" targetFramework="4.0" />
</system.web>
<system.webServer>
    <defaultDocument enabled="true">
        <files>
            <add value ="DuelPortalOnlineTestPage.html"/>
            <add value="ClientBin/DuelPortalOnline.xap"/>
        </files>
    </defaultDocument>
</system.webServer>
<system.serviceModel>
    <services>
        <service name="DuelPortalOnline.Web.ChatService" >
            <host>
                <baseAddresses>
                 </baseAddresses>
            </host>
            <endpoint address="" binding="pollingDuplex" bindingConfiguration="myPollingDuplex"
                contract="DuelPortalOnline.Web.ChatService"  behaviorConfiguration="duplexBehavior" />
            <endpoint address="mex" binding="mexHttpBinding" contract="IMetadataExchange" />
        </service>
        <service name="DuelPortalOnline.Web.SQL_Webclient_Service">
            <endpoint address="" binding="customBinding" bindingConfiguration="DuelPortalOnline.Web.SQL_Webclient_Service.customBinding0"
                contract="DuelPortalOnline.Web.SQL_Webclient_Service" />
            <endpoint address="mex" binding="mexHttpBinding" contract="IMetadataExchange" />
        </service>

    </services>
    <extensions>
        <bindingExtensions>
            <!--
            <add name="pollingDuplex" type="System.ServiceModel.Configuration.PollingDuplexHttpBindingCollectionElement,System.ServiceModel.PollingDuplex, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />
            -->
        </bindingExtensions>
    </extensions>
    <behaviors>
        <serviceBehaviors>
            <behavior name="duplexBehavior">
                <serviceMetadata httpGetEnabled="true"  />
                <serviceDebug includeExceptionDetailInFaults="true" />    
            </behavior>
        </serviceBehaviors>
    </behaviors>
    <bindings>
        <customBinding>
            <binding name="DuelPortalOnline.Web.SQL_Webclient_Service.customBinding0" >
                <binaryMessageEncoding />
                <httpTransport />
            </binding>
        </customBinding>
        <pollingDuplex>
            <binding name="myPollingDuplex" duplexMode="MultipleMessagesPerPoll" >
            </binding>
        </pollingDuplex>
    </bindings>
    <serviceHostingEnvironment aspNetCompatibilityEnabled="true"
        multipleSiteBindingsEnabled="true" />
</system.serviceModel>